Singlet oxygen production in herbicide-treated photosystem II

Photo-generated reactive oxygen species in herbicide-treated photosystem II were investigated by spin-trapping. While the production of (OH)-O-. and O-2(-.) was herbicide-independent, O-1(2) with a phenolic was twice that with a urea herbicide. This correlates with the reported influence of these herbicides on the redox properties of the semiquinone Q(A)(-.) and fits with the hypothesis that O-1(2) is produced by charge recombination reactions that are stimulated by herbicide binding and modulated by the nature of the herbicide. When phenolic herbicides are bound, charge recombination at the level of P(+.)Pheo(-.) is thermodynamically favoured forming a chlorophyll triplet and hence O-1(2). With urea herbicides this pathway is less favourable. (C) 2002 Federation of European Biochemical Societies. Published by Elsevier Science B.V. All rights reserved.